Estimated probability of a severe outcome (with 95% confidence intervals) for an individual with the mtDNA m.8993T>G or m.8993T>C variant, based on the mutant load of the individual. A severe outcome is defined as severe symptoms of NARP or Leigh syndrome and probability is calculated using a logistic regression model described in White et al [1999a]. Note that the risk of developing severe symptoms is minimal — below the approximate threshold values of 50% for m.8993T>G and 75% for m.8993T>C.